Introduction to Waterproof Breathable Membrane Technology
How does waterproof breathable (WP/B) technology as used in outdoor apparel really work? The standard answer is often a simple one: "little tiny pores in the fabric are too small to allow liquid moisture (rain) to pass through, but are large enough to allow perspiration vapor (sweat) escape." Are you aware that this is not how Gore-Tex and most polyurethane coatings really work? This myth has been propagated for so many years it has now become cliche. Herein, we discuss how WP/B technology really works and which membranes actually do allow for the diffusion of vapor molecules.
Herein, we'll address:
· How various waterproof breathable technologies function
· Performance differences between waterproof breathable technologies
· Gore-Tex laminate fabric performance
· Polyurethane hydrophilic monolithic laminate fabric performance
· eVENT fabric performance
· 3M Propore fabric performance
· Entrant G2 -XT fabric performance
· Test methods for measuring breathability
· Relevance of this technical information to practical field application (summary)
· Rain shell usage and selection guidelines
Breathability, or water vapor transmission rate (WVTR), is the ability of a fabric to transport water vapor from one side of the fabric to the other. Nominally, the greater the WVTR, the faster water vapor moves from the inside of your garment to the outside, and the less moisture you’ll accumulate while exercising. But the tests mentioned in this article measure the WVTR of only a small sample of fabric in a flat configuration under controlled laboratory conditions. The results from these tests should only be used as a tool for comparing breathability between fabrics, and should not be construed as predictors of field performance of garments. Note: many breathability tests are not limited to evaluating WP/B fabrics. They can also be used to assess the performance of uncoated fabrics such as those used in soft shells and wind shirts.
Sometimes called dry thermal resistance, this is the insulating value of a shell. Even a thin layer of fabric has some insulating value which can contribute to the wearer’s comfort or discomfort. On a warm day even a very thin microfiber wind shirt can make you uncomfortable: try running on an 80 degree day in a windshirt layered over your normal T-shirt and compare it to just the cotton T-shirt you normally wear! Since sweating is a response to heat, you’ll sweat more, which makes garment breathability even more important.
This is a measure of a how much air can pass through a fabric at a given pressure. It is a significant factor in garment comfort at high exertion levels. Air permeability is usually measured in cubic feet per minute. WP/B garments have very little or no air permeability and this contributes to user perception of clamminess and the buildup of interior moisture in the clothing system. Garments such as thin uncoated nylon wind shirts or stretchwoven soft shell jackets can have significant air permeability. The convective air and vapor flow through the air permeable soft shell fabric is a major reason why soft shells are so comfortable - they can transport water vapor at much higher rates than any WP/B garment. Conversely, lack of air permeability is the primary reason why WP/B fabrics are not so comfortable during high exertion. Most lab breathability tests do not account for moisture transport and cooling resulting from the air permeability of the fabric. Therefore, even though eVENT and Schoeller Drysking Extreme may have the same MVTR in a particular test (see Figure 1), field performance will be dramatically different - the more air permeable Schoeller fabric will result in significantly less moisture accumulation in the clothing system.
Hydrophobic Microporous Membrane
A water repelling membrane with continuous pores through the membrane that allow water in gas form to pass through the membrane. The membrane’s waterproofness depends on the water repellence of the membrane’s material and not the pore size. Hydrophobic microporous membranes include eVENT, a few new polyurethane coated fabrics, 3M Propore, and “first generation” Gore-Tex (not in use for outdoor fabrics). First generation Gore-Tex is also referred to as an “expanded PTFE membrane” in some charts and is often used as a reference standard for breathability testing.
Hydrophilic Monolithic Membrane
A water absorbing membrane that is solid. There are no continuous pores through the membrane that allow water in gas form to pass through. Moisture moves through the membrane by absorption into the membrane on the inside face, transport through the membrane by solid state diffusion, and finally, evaporation on the outside face of the fabric. The membrane’s waterproofness depends on a solid membrane that physically blocks liquid moisture. Hydrophilic monolithic membrane fabrics include most polyurethane coated fabrics. “Second generation” Gore-Tex fabrics (Teflon with a polyurethane coating) also use hydrophilic monolithic membranes. These include the Gore-Tex membranes used in today's outdoor fabrics.
How Gore-Tex Really Works (The Answer May Surprise You)
First we’ll cover Gore-Tex, the (aging) granddaddy of all waterproof breathable membranes. More than 25 years ago, W. L. Gore created a thin membrane of expanded PTFE (a.k.a., "Teflon"; with a physical structure akin to a filamentous web). This membrane was waterproof but let water in gas form pass through (a micro-porous membrane). Presto, a waterproof breathable membrane!
How does it work? The common myth is that the spaces between the PTFE filaments are large enough to let water in a gas form pass through, but small enough to prevent water in liquid form to pass through. This is not exactly true. In fact, there is plenty of room for liquid water to pass through the spaces between the PTFE filaments!
More correctly, the surface of the PTFE membrane is at a high surface energy level (in lay terms, this means that it is extremely water repellent, or hydrophobic). Water in liquid form is repelled with such force (resistance to capillary wicking) that it takes a great deal of pressure to push it through the PTFE membrane. Water at a pressure resulting from a heavy rain doesn’t begin to reach this level. Conversely, if something were to take the membrane’s surface to a lower energy level, the water repellency would be lost and water would pass through the spaces between its filaments via capillary wicking. This is “first generation” Gore-Tex fabric and exclusively relies on the hydrophobicity of the microporous membrane for its water resistance.
When first generation Gore-Tex appeared in rainwear, it worked great - for awhile. However, customers began returning garments that had begun to leak. Here's what happened: body oils, detergents, dirt and other common chemicals contaminated the PTFE membrane from inside. The membrane’s surface was initially effective enough to repel water, but it couldn't repel these contaminants. Contaminant adsorption to the PTFE surface decreased its surface energy, creating pore channels with poor water repellency that wicked moisture through the membrane’s structure - not exactly the functionality you want in a waterproof garment. The result: the garment leaked. (It is worth noting that a fully expanded PTFE membrane can have an air porosity as high as 80% - so there is plenty space between the filaments for water to wick.) Thus, both surface energy as well as pore size are important factors affecting the waterproofness of a membrane. Gore learned the hard way the consequences of failing to recognize the long-term consequences of contaminant fouling of expanded PTFE.
Gore engineers tried to design a technology to keep the membrane free of contamination. The result: cover the PTFE surface with another membrane to protect it from contamination. The choice for this second membrane was polyurethane (PU). But how was water vapor to get through the fabric, since the pores of PU in its natural state are not large enough to be permeable to water - in either liquid or gas form?
The trick was to chemically modify the PU to be water absorbent (hydrophilic). Water in gas or liquid form is adsorbed by the PU membrane when it comes in contact with the membrane’s surface. Once adsorbed to the membrane surface, individual water molecules are then transported via solid state diffusion to the outer surface of the PU membrane. Diffusion is a chemical process where a substance at high concentration is transported to a region of lower concentration. In the case of solid state diffusion of water through a PU membrane, individual water molecules in their liquid form (H2O) creep their way through the PU matrix, driven by the concentration gradient formed by a high concentration of water on the wet inner surface to a low concentration of water on the dry outer surface. Once the water molecule reaches the outer surface of the PU membrane, it can then evaporate (become a gas phase molecule) and transport via gas phase diffusion through the PTFE membrane. It is important to note that the PU membrane is solid (monolithic). There are no spaces for a gas or water to pass though it. Water in either gas or liquid form is transported on a molecular level through the solid PU membrane (solid state diffusion). We refer to this type of PU membrane as a hydrophilic monolithic membrane.
So far so good. Gore had prevented contamination and they engineered breathability in the form of water molecule transport by solid state diffusion thought the PU membrane. Now the real problems start: the solid PU membrane moved water at a rate significantly less than the original PTFE membrane it covered! Thus, Gore made this membrane as thin as possible (see durability issues in the next paragraph), achieving a level of performance that was sufficient to bring the technology to market and they marketed Gore-Tex with its new PU membrane protection. This use of a hydrophobic micro-porous PTFE membrane covered by a PU hydrophilic monolithic membrane is often termed “second generation” Gore-Tex. Gore has made improvements in both weight and breathability relative to first generation Gore-Tex with the development of fabrics such as PacLite III and XCR, but they still employ the basic PU-PTFE membrane technology used in second generation Gore-Tex.
One final issue: PU-PTFE Gore-Tex membrane was delicate, resulting from an ultrathin PU layer, and had to be protected. Wear abuse causes pinholes and cracks to develop in the membrane and create leakage channels. Protection for the Gore-Tex membrane was accomplished by either adding a free hanging nylon liner on the inside of the garment or (more commonly used nowadays), by laminating a protective layer of polyester tricot to the inner surface of the garment. The latter type of construction is used in what is commonly known as “3-layer” fabric. Typically (unless laminated to an extremely light face fabric), three-layer fabrics are bulky, heavy and compress poorly. Gore’s 2.5-layer PacLite III mitigates some of these problems by substituting a textured surface pattern on the inner face of the WP/B membrane in lieu of the heavier and bulkier tricot inner liner.
Polyurethane Membranes (Hydrophilic Monolithic)
If you’ve made it this far, you will astutely note that it seems as if the PTFE is doing nothing, with the PU membrane acting as a bottleneck to moisture transmission. This is almost true - with a caveat. It turns out that the highly filamented surface of the PTFE membrane provides sufficient roughness to bond (laminate) a very thin PU membrane without imperfections that lead to leakage. If you try to laminate a PU membrane to another surface (e.g., nylon), you must use a thicker membrane - up to three times as thick as the PU membrane Gore laminates to their PTFE membrane.
Why is the thickness of the PU membrane important? Solid state diffusion is a relatively slow process that is directly proportional to the concentration gradient and intrinsic diffusion properties and inversely proportional to the membrane thickness. The thinner the membrane, the faster water moves through it. Gore-Tex, with its very thin PU membrane, offered faster moisture transport (better breathability) than the thicker PU membranes laminated to non-PTFE surfaces. Thus, at the time of second generation Gore-Tex, Gore definitely had a technological edge over competing technologies that employed PU laminated directly to nylon.
Most of the early hydrophilic monolithic PU fabrics with their thicker membranes could not match the breathability of Gore-Tex. Over the years, they have made improvements in intrinsic diffusion properties and thinner membranes, and have closed the gap. Some even perform a bit better than second generation Gore-Tex. Still, most non-Gore PU garments, while close, haven’t quite reached the breathability of the best Gore fabrics such as PacLite III and XCR. Gore technology is improving as well.
One exception is Entrant G2-XT by Toray, a nylon fabric that relies only on the performance of a PU membrane for moisture transport. Tests at both the Army’s Natick Labs and Kansas State University’s Institute for Environmental Research indicate that Entrant G2-XT is more breathable than Gore's best fabrics but less breathable than eVENT fabrics. At varying humidity levels, Entrant G2-XT behaves more like eVENT (a hydrophobic micro-porous membrane) than hydrophilic monolithic PU fabrics. We’ll discuss Entrant G2-XT fabric in its own section later. For now, see Figure 1 and Table 1 for a performance comparison of Entrant G2-XT to other fabrics.
Advantages of PU-Only Garments
Fabrics that employ only a PU membrane do have some major advantages PU-PTFE membranes like Gore-Tex. A PU-only membrane is more durable than a PU-PTFE construction and does not require an additional liner of polyester tricot for protection. Thus, PU only garments are lighter, more flexible, and more compressible than Gore-Tex garments. The lightest PU-only garments are in the range of 8 oz (225 g) for a seam-sealed, hooded, full zippered jacket (e.g., the 2003 Montane Superfly). By comparison, the lightest Gore-Tex jackets using 2.5 layer PacLite III are just under 12 oz (340 g) (e.g., 2003 GoLite Phantom). Also, PU-only technology is cheaper to manufacture and typically doesn’t have the added cost of having to support a large marketing budget! Thus, a PU-only garment costs significantly less than a similar Gore-Tex garment. You can get a fully featured PU-only rain jacket such as the 2003 Red Ledge Thunderlight Parka for only $50. A similar Gore-Tex PacLite III or XCR parka will cost around $225 to $300 - or higher.
Finally, PU has enough elasticity to retain its function and durability when combined with stretch face fabrics. One example of a WP/B stretch fabric is Pearl Izumi’s Stretch AmFIB. Manufacturers use these stretch WP/B fabrics for socks, gloves, tights, and stretch panels in a variety of garments. PTFE based laminates like Gore-Tex and eVENT have low elasticity and are most commonly found only on non-stretch fabrics. Manufacturers making garments out of Gore-Tex and eVENT may use panels of stretch PU fabrics to enhance range of motion in these garments while maintaining a trim fit. Stretch Gore-Tex constructions are available, but they remain expensive and are not very common.
What if you could use the original expanded PTFE membrane and not have to cover it with a PU membrane? You’d have a waterproof breathable membrane that would move moisture approximately 30% to 200% faster than Gore-Tex XCR*. BHA Technologies has developed such a membrane, branded as eVENT. Sounds like first generation Gore-Tex, doesn't it? Here's the difference: BHA engineered the PTFE membrane to be oleophobic, and thus, highly resistant to contaminant fouling. The eVENT PTFE membrane repels most of the common oily contaminates that originally plagued the first generation Gore-Tex membrane. Thus, eVENT does not need a breathability-inhibiting PU membrane to protect it from contaminants.
* Results from independent testing: US ARMY Natick Labs, Dynamic Moisture Permeation Cell (DMPC), ASTM F2298 approved June 2003.
Figure 1 - Breathability performance of some common outdoor fabrics. Higher values indicate better performance: Test results from US Army’s Soldier Systems Center, Natick, MA using a Dynamic Moisture Permeation Cell (DMPC), ASTM F2298. Courtesy of Phillip W. Gibson.
With unimpeded spaces between fibers (a micro-porous membrane), eVENT’s moisture transmission behavior is more like an uncoated woven or knit fabric. From Figure 1, you can see how closely eVENT matches the performance of Schoeller’s Dryskin Extreme, a stretch woven fabric that has no WP/B membrane (note that the air permeability effects of a fabric on moisture transport are eliminated in this test; if air permeability is considered, much higher moisture transmission rates would be measured for Schoeller Dryskin Extreme).
eVENT’s membrane structure is important in two ways. First, molecules in their gaseous state (including perspiration vapor) can pass through the pore channels between the membrane’s fibers, a process that is capable of moving significantly higher fluxes of moisture than what is possible for by a solid layer of PU (as in Gore-Tex). Second, and more important, eVENT passes moisture equally well at both low and high humidity levels, evidenced by the independence of eVENT’s moisture transmission rate on the humidity level (see Figure 1). At a 70% mean humidity level, eVENT transports moisture about 30% faster than XCR and about 70% faster than standard Gore-Tex and the best PU membranes. But at a 30% mean humidity level, eVENT transports moisture about 200% faster than XCR and about 250% faster than standard Gore-Tex and the best PU membranes.
Advantages of eVENT Garments
Why is faster moisture transport at low humidity important? If moisture can be moved out of a clothing system soon after humidity begins to build (low humidity levels), the risk of moisture vapor accumulation, condensation, and wetting of clothing worn underneath the shell is minimized. The ability of a shell fabric to pass moisture at a high rate at low humidity levels, then, increases the time at which the shell can be worn during periods of moderate exertion levels.
But now consider sustained or higher exertion levels, resulting in perspiration that overwhelms the ability of a garment to breathe, and your clothing starts to get wet from the accumulation of perspiration. Since eVENT moved moisture faster and started moving it earlier, you’ll maintain drier clothing longer than if you wore a PU garment. More important, when your exertion level lowers and the internal humidity level in the garment decreases, eVENT will continue to move moisture at a higher rate than PU, which ultimately contributes to faster drying times for your clothing.
In cool and dry environments, such as winter camping or high altitude mountaineering, humidity levels can be fairly low - a situation that would favor eVENT. This is an environment where getting your clothing damp may have serious consequences for staying warm. This may be even more important for non-clothing applications like tents and bivies. Here, relative humidity may remain low but the need to move moisture out and reduce condensation is high. Some manufacturers are starting to use eVENT in their single wall tents and bivy sacks, including Integral Designs, Exped, and Bozeman Mountain Works.
One final advantage for eVENT garments is that they are easy to clean. Gore recommends that you wash Gore-Tex with special (and expensive) cleaning products (if you don't, you run the risk of reducing the hydrophilicity of the PU membrane - which could compromise its ability to transport moisture). You can wash eVENT pretty much the same way you wash your regular clothes - use a mild liquid soap (detergent-free) and a 2x rinse cycle.
Venting a Garment
Of course, any discussion of breathability must also consider garment ventilation features. Previously, we've addressed the relationship of ventilation and breathability in The Science of Breathability and Its Impact on Raingear Selection and Use and High Exertion Moisture Accumulation in Rain and Wind Shells. While shell fabric breathability contributes to moisture transport, shell ventilation design features are also important for controlling moisture accumulation in your clothing system. Further, how you use your shell – and its ventilation options – is vital. A vented jacket with a reasonably breathable shell used properly may still outperform a poorly vented jacket with a more breathable shell fabric. eVENT’s breathability helps level the playing field some, and allows for more efficient designs with less ventilation features, resulting in a shell that is simpler to use and lighter in weight. As of press time, the only eVENT jacket on the market that employed pit zips was the Gill Adrenaline, a hoodless cycling jacket. A newcomer to the US market is the 66° North Glymer jacket - which uses mesh-backed front pockets to aid ventilation. None of the other eVENT jackets on the market employ pit zips, core vents, or mesh-backed pockets for ventilation, instead relying on eVENT’s inherent breathability to move moisture out of the clothing system.
At high exertion levels, it isn’t difficult to overload the breathability capacity (moisture transport flux) of the more breathable WP/B fabrics like Gore-Tex, high performance PU membranes, and even microfiber windshirts made with uncoated woven fabrics. eVENT is no exception. In our field testing of un-vented eVENT jackets at very high aerobic levels (>80% max heart rate) in windy and dry conditions at 40 °F (4 °C) our base layer (the only clothing worn under the jacket) was fairly wet after an hour. This is not to say that eVENT performed poorly – on the contrary, we feel that eVENT has excellent breathability and jackets made with eVENT are among the most breathable we’ve tested. This is just reminder to readers that even the most breathable of WP/B fabrics can be overwhelmed during sustained periods of high exertion. As activity intensity and duration increases, the benefit of more air permeable fabrics, such as Schoeller Dryskin and other “soft shell” stretchwovens, or garments with ventilating features such as pit zips, becomes more beneficial.
Drawbacks of eVENT Garments
So what are the drawbacks of eVENT?
First, the eVENT membrane is delicate and requires a tricot liner for protection. This adds weight and bulk to the garment.
Few manufacturers currently use eVENT, so product styles are limited. In addition, ultralight, three-layer eVENT fabric constructions are not yet available. Current eVENT jackets with a hood are typically 15 to 21 oz (425 to 600 g). We are currently testing a 15 oz (425 g) eVENT version of Montane’s Super-Fly jacket scheduled to go to market in Spring 2005, and should be the lightest eVENT jacket on the market. By comparison, the PU version of this jacket weighs only 8.5 oz (245 g). Given the delicate nature of eVENT’s membrane it is unclear when a lighter 2.5 layer eVENT technology will be released. We have some reports that at least one manufacturer is pursuing a 2.5 layer eVENT jacket, although it will be at least a year or more before it goes in production. BHA technologies has authorized a few manufacturers to use 2-layer eVENT fabrics in sleeping bag and down jacket shells, including Feathered Friends, and Bozeman Mountain Works will launch synthetic fill clothing and sleeping bags using a new 1.6 oz 2-layer eVENT fabric in its 2005 product line.
Because of the breathability of eVENT, manufacturers have not built the same venting options (core vents, pit zips, and mesh backed pocket) on eVENT jackets as on their PU jackets. Manufacturers are trying to maintain the waterproofness of their eVENT shells and to reduce weight by removing features like extra zippers and vented openings. This trend is indicative of three forces: consumer demand for lightweight gear, manufacturer’s desire to keep costs in check, and the lack of availability of lighter 3-layer eVENT fabrics. An eVENT jacket with versatile ventilation options might be the holy grail of rain jackets, but this market niche won’t be solidified until a full-featured eVENT jacket can be built for less than 12 ounces, the current standard of lightweight full-featured rain jackets made with Gore-Tex XCR (e.g., 2004 Mont-Bell Torrent Flier), PacLite III (e.g., 2003 GoLite Phantom), or number of PU-only fabric garments (e.g,. Marmot Precip). The current standard of sort-of-full-featured eVENT rainwear is made by Gill: they have a 16 ounce hoodless eVENT jacket with pit-zips. It is a cycling jacket, but should be suitable for hiking, running and backpacking as well. Lack of a hood will limit its penetration into the hiking and climbing markets, however. 66° North, an Icelandic company, recently debuted a hooded eVENT alpine climbing jacket with vented chest pockets at the 2004 Outdoor Retailer Winter Market, but its weight in a size men's L will still exceed the 18 ounce mark.
Like Gore-Tex, eVENT garments are expensive. Expect to pay $200 to $350+ for an eVENT jacket. The high cost of eVENT fabrics has manufacturers adding less expensive fabrics, such as stretch PU panels in their garments. While aiding mobility and allowing a trimmer fit, the stretch fabrics are heavier and less breathable than eVENT.
3M Propore: Microporous Polypropylene Laminate
There is another material in use for rainwear that exhibits breathability performance similar to eVENT: 3M Propore, a microporous polypropylene fabric. Propore uses a WP/B microporous polypropylene membrane laminated to a nonwoven polypropylene fabric (the yellow 2-layer fabric used in the Rainshield's O2 Rainwear product lines) or a WP/B microporous polypropylene membrane laminated between two nonwoven polypropylene fabrics (the blue 3-layer fabric used in Rainshield's Sporting product lines). Propore breathes equally well at low and high humidity levels. Propore is another extremely breathable fabric with DMPC water vapor transmission rates (breathability) that exceed Gore-Tex XCR (Figure 2). Unlike eVENT and Gore-Tex XCR, the polypropylene based technology used in Propore is very inexpensive and the fabric is very light. It is even lighter and less expensive that PU WP/B garments. The hooded Rainshield Propore jacket weighs 4.7 oz (133 g) and costs only $30! It’s no wonder that Propore is a favorite cult choice of long distance hikers.
Figure 2 - Breathability performance 3M Propore fabric vs Gore-Tex fabrics and a reference PTFE membrane. Higher values indicate better performance: Test results from US Army’s Soldier Systems Center, Natick, MA using a Dynamic Moisture Permeation Cell (DMPC), ASTM F2298. Courtesy of Phillip W. Gibson.
The major disadvantage of Propore is that it is not a durable fabric and is especially prone to punctures and tears. For most users it is limited to trail use and more benign environments. It is ill-suited to serious bushwhacking, scrambling or climbing. Some members of the BackpackingLight.com review staff have quite a lot of off-trail (alpine hiking) experience with Propore garments. With care, they’ve found them to be quite serviceable, even for some cross-country travel and light mountaineering. Propore garments as manufactured by Rainshield are done so to achieve a price point. Consequently, they offer few features, poor fit and styling, and they lack the manufacturing quality of higher end outdoor specialty soft goods. There is certainly room – and demand – for a well-designed Propore rain jacket that is better suited for extended use.
3M Propore is not the only polypropylene-based fabric on the market. Similar to three-layer Propore in construction (with a membrane sandwiched between two layers of nonwoven polypropylene), the fabric used in Frogg Toggs is not as breathable, based on both field observations and laboratory testing (ASTM E96) performed by BackpackingLight.com (see M Raingear Roundup). Frogg Toggs have achieved a cult following among the lightweight backpacking community, and have penetrated that market more deeply than Rainshield, owing to more style options, higher construction quality, and better fit.
Entrant G2 XT (Hydrophobic Microporous WP/B (Polyurethane)
According to the results from the Army’s Natick Labs and Kansas State University’s Institute for Environmental Research, Figure 1 and Table 1 respectively, Entrant G2 XT is more breathable than Gore-Tex fabrics but not quite as breathable as eVENT. In Natick’s DMPC tests, the flat line of breathability vs. relative humidity for Entrant G2 XT is similar to that of eVENT and suggests the use of a hydrophobic micro-porous membrane. Our suspicions were confirmed after speaking with Shunichi Higashi, General Manager of Textiles for Toray America at the 2004 Outdoor Retailer Winter Market in Salt Lake City. He confirmed that Entrant G2 XT employs a hydrophobic micro-porous membrane. We are a bit confused as to why we haven’t heard more about this highly breathable fabric. It is used predominately in bicycling, motorcycling and ski shells (with very few hiking / backpacking garments using the fabric). Market penetration is solid in Europe (especially the UK) and poor in the US. As this technology evolves, it has the potential to bring eVENT-like performance into lighter, less expensive garments.
Laboratory Fabric Breathability Tests: Which one? Do they mean anything?
Much of the confusion about the breathability performance of various membranes and fabrics is fostered by the multitude of breathability test methods. Thus far, no standard test exists for measuring the breathability of a waterproof or water-resistant fabric. Membrane manufactures are likely to select a test method and/or the conditions for that test method that allows their product to outperform their competitors.
Even the best lab tests, carefully performed, provide only the roughest indication of garment field performance.
It is important to understand that these tests are performed on small pieces of fabric, in a laboratory, and under a limited set of controlled conditions - an environment that is not exactly scalable to assessing the performance of a garment in the field.. According to two thought leaders in the field of fabric performance testing, Dr. Elizabeth McCullough (co-director for the Institute for Environmental Research, Kansas State University) and Dr. Phillip Gibson (US Army Soldier Systems Center), test results on fabrics are just that: tests on fabric. At best, these tests can only approximate the field performance garments. McCullough and Gibson both point out that more sophisticated tests that use moving and sweating mannequins clothed with realistic apparel systems and subjected to blowing wind are more accurate indicators of clothing performance, but still fall short of predicting actual field performance on human subjects. Mannequin tests are also expensive, difficult to execute, and provide data that can be interpreted with great latitude. Consequently, they are not likely to gain favor in breathability standardization anytime soon, and will remain primarily as a research tool.
Even if testing were performed on human subjects, many variables must be taken into account: individual metabolic variability, individual perspiration level, personal fitness, activity level, what garments are worn under the shell, shell venting characteristics (e.g. pit-zips), garment fit, whether or not the shell “pumps” air (which is governed by fit, ventilation, and body motion), the type of activity performed, wind speed and direction, outside temperature, precipitation levels, etc. This list could go on! Clearly, there is no standard method or measure that can be used to predict the comfort level of rainwear garments on a human subject in realistic field conditions. There are simply too many variables.
The following table summarizes the test results from some of the standard fabric breathability tests used by researchers and industry. Tests were performed at Kansas State University’s Institute for Environmental Research.
Table 1. Results from Different Water Vapor Transmission/Resistance Tests
Performed on WR/B & WP/B Shell Fabrics
Table provided from Elizabeth A McCullough Myoungsook Kwon and Huensup Shim "A comparison of standard methods for measuring water vapour permeability of fabrics," Institute Of Physics Publishing, Meas. Sci. Technol. 14 (2003) 1402-1408 (Used with Author's Permission).
One could write thousands of pages on tests that measure fabric breathability. One quickly gets lost in abstruse physics and experimental procedures. Even a fairly simple explanation of the standard fabric breathability tests is beyond the scope of this article. For now, we’ll focus on just two tests, the Sweating Hot Plate (SHP) or “Skin Model”, and the Dynamic Moisture Permeation Cell (DMPC) tests. There are at least four other tests in common use and many variations of test conditions. Needless to say it is extremely difficult to compare results from one type of test to another since they do not always use the same units or give comparable numeric results. One should even be careful in comparing the results of the same test between labs since the apparatus and test conditions can vary. Finally, one must consider the inherent variability (scatter) in data between tests, even at the same lab using the same test setup.
The bottom line: You should view fabric breathability performance numbers quoted by a garment manufacturer with some skepticism. The design of a garment and how you use it will likely have more impact on its field performance than the lab measured breathability of its WP/B fabric. The most reliable breathability data can be interpreted by comparing results using the same test method on different fabrics from an independent laboratory, such as those quoted in Figures 1 and 2 and Table 1 herein.
Sweating Hot Plate (SHP) Test
Both Gore and McCullough note that only one test, the Sweating Hot Plate test, has been correlated in some way to “field performance” on human subjects. This is not to say that other tests lack the potential to be correlated to field performance - it just hasn’t been done yet. The correlation of the SHP test to field performance was performed at Hohenstein labs. McCullough tells us that participants wore only a light base layer, and were covered with rainwear that was sealed at the neck, wrists and ankles. During the test, they reported on how comfortable they felt while performing various activities. (We’ll discuss Hohenstein’s interpretation of these results later in the article.)
The other advantage of the SHP test is that it can test both the thermal resistance (insulation) of a fabric as well as its resistance to moisture transfer (breathability). Advocates of the test feel that this is a good simulation of how people perceive discomfort (getting too warm and getting too clammy). The thermal insulating value of the fabric contributes to getting too warm since even a thin piece of fabric has some insulating value and stops air movement away from the body. The moisture transport resistance (or lack of breathability) also makes people warm since sweat no longer evaporates to cool the body.
The SHP test does have a few limitations. It tests breathability at high humidity levels (usually 100% relative humidity on the “inside” of the fabric). As such, it favors the performance of hydrophilic PU fabrics (including Gore-Tex) since they have much higher moisture transport rates at high humidity levels. Advocates of the SHP test point out that this is a reasonable approximation of a sweating human body at a high activity level.
Others note that one might want to tell how a fabric performs at many activity levels. Or as Gibson says, “for hydrophilic (PU) membrane laminates, the single point tests (one set of humidity levels) miss the change in properties of these materials at different humidity conditions, and can be misleading. Gore-Tex and other hydrophilic (PU) membrane laminates perform best in very humid environments, which is good, but it also implies that they’ve failed by allowing you to start sweating in the first place.”
As fabrics get more breathable (e.g., Gore-Tex XCR and eVENT), their resistance to moisture transport gets very low - close to the resistance of a sweating hot plate without any fabric over it. This makes it difficult (but not impossible) to resolve differences between very breathable fabrics. McCullough says that for her lab, SHP test breathability differences of 10% between fabrics in the “extremely breathable” range may not be meaningful. Gibson says “It is true that because of the nature of the test, you can start to get buried in the noise when you approach the bare plate value, but if you’re careful, you can discriminate pretty well in the extremely breathable range.”
Finally, the SHP, when conducted per the guidelines of ISO 11092, calls for both sides of the fabric to be held at a constant temperature of 35 °C. Gibson and Natick believe that a temperature differential is a more realistic way to perform this test. Gibson runs the SHP test with the inner surface of the fabric at a warmer temperature than the outer surface. This mimics the warmer environment on the inside of your clothing and the cooler air outside your clothing, and is a more realistic predictor of field performance, when raingear is usually worn in cooler conditions.
Dynamic Moisture Permeation Cell (DMPC) Test
Gibson and Dr. Fred Wilson, BHA Technologies' and eVENT's chief technology engineer, emphasize that the Dynamic Moisture Permeation Cell (DMPC) test can measure fabric breathability at a variety of humidity levels. In the DMPC test, the relative humidity (RH) on both sides of the test fabric can be independently controlled. You can test with 20% RH on the inside of the fabric and 10% on the outside all the way up to 100% RH on the inside of the fabric and 90% on the outside.
PU fabrics perform best with high humidity levels on the “inside” of the fabric. By comparison, some WP/B fabrics like eVENT and Propore maintain a high level of breathability even at low humidity levels. As pointed out earlier in this article, there are reasons why improved breathability at lower humidity might be important for garment performance. Gibson says, “I would agree that for clothing use, that the rankings down on the humid end are probably most appropriate if you had to pick one condition. But I think that the less humid end of the curve is also important for the period before you start sweating, and for the situation where you are trying to dry your clothing, boots, or gloves.”
As mentioned earlier, another advantage of the DMPC test is that it can measure the air permeability (the opposite of windproofness) of the test fabric. While air permeability is not a characteristic of WP/B membranes (most are nearly or completely impermeable to air), air permeability is the primary reason why soft shell garments are so comfortable. The air permeability of a nylon wind shirt or stretchwoven jacket is why they are more pleasant to wear than WP/B garments in anything short of a sustained rain. Note the high breathability ratings of Schoeller’s Dryskin Extreme on the DMPC tests (Figure 1). The more air permeable Schoeller Dynamic would be off the charts in the same test. An active outdoorsman, Dr. Gibson uses soft shell garments as outer shells for almost all of his endeavors.
Glenn W. Crowther, of W. L. Gore contends that the disadvantage of some of the newer tests like the Dynamic Moisture Permeation Cell (DMPC) as per ASTM F2298 is that they have not been correlated to the performance of waterproof breathable garments on an exercising human body. For Gore, it is unclear if the results from the DMPC test mean anything about “real world” or “in field” performance of a garment.
Crowther says Gore favors the SHP test. They believe that the SHP test best replicates the performance of a fabric covering a sweating human body. They point out that Hohenstein labs in Germany has correlated SHP test results to human subject testing or “in field” use. Gore also favors the SHP test because it has been adopted as an international standard and it has been in use for some time. It was developed in collaboration with independent labs, including the Army’s Natick labs in the U.S. and Hohenstein labs in Germany. Anyone can send samples to these labs for unbiased testing of breathability.
Discerning the extent to which a test design remains free of manufacturer influence is very difficult. The scientific committees responsible for developing, refining, validating, and approving such test methods are collaborative efforts of individuals from both industry and academia. A reliable source, and committee participant, who prefers to remain anonymous, states, “conflict of interest is nearly impossible to remove from the test method approval process. Committees for the most part behave respectably, but individual members, especially those represent the major financial interests of for-profit corporations, certainly have their own agenda, and exercise tremendous influence on the adoption of test methods.” Suspicion of such agenda-(and profit)-driven tainting of the test method approval process appears to be prevalent in other industries as well, including and especially, test methods used to evaluate the efficacy of pharmaceuticals, biomedical devices, and chemical coatings - three industries with potentially huge profit margins on products brought to market and endorsed by an internationally-adopted test method. Does this mean the test methods are not valuable? Of course not. But buyer beware: test results alone will not tell the whole story, and may be skewed to unfairly favor a particular manufacturer's technology.
Ranking of Fabric Field Performance Using the SHP Test
Gore and Hohenstein labs have developed a ranking system based on a fabrics Ret performance in the SHP test:
· Ret 0 to 60 = "extremely breathable" (e.g., Gore-Tex XCR, PacLite III, eVENT)
· Ret 60 to 130 "very breathable" (e.g., Gore-Tex Classic, most PU laminates)
· Ret 130 to 200 "breathable"
· Ret 200 to 300 "slightly breathable"
· Ret >300 "not breathable"
Note: a Ret is a measure of resistance to moisture transport. A lower Ret means a fabric has less resistance or is more breathable, i.e., lower Ret values indicate more breathable fabrics.
1. View laboratory-based fabric breathability test numbers as reported by manufacturers with caution. They may not have all that much to do with how you feel wearing a garment made from that fabric, anyway, and manufacturers may have selected the test method - and test conditions - to favor their fabric.
2. There are a lot of good WP/B fabrics out there. For less breathable fabrics, garment design (including ventilation features) and how you use the garment increase in importance for maintaining comfort during active conditions.
3. Polyurethane only fabrics and bicomponent PTFE fabrics (i.e., those with a hydrophilic PU membrane like Gore-Tex) breathe better at higher humidity levels. In other words, they transport moisture better when it’s warm and moist inside your garment.
4. There is a new class of WP/B breathable fabrics with hydrophobic micro-porous membranes like eVENT, Propore and Entrant G2 XT that breathe better at all humidity levels than the best Gore-Tex fabrics. These new fabrics are extremely breathable even at low humidity levels, unlike Gore-Tex. They start venting before it gets warm and clammy inside your garment. According to some researchers, the enhanced breathability of these fabrics may increase your comfort over conventional Polyurethane and PU/PTFE fabrics like Gore-Tex.
5. Fabric air permeability plays a significant role in garment comfort at high exertion levels. Most WP/B fabrics are not air permeable and this contributes to them being too warm and clammy at moderate to high exertion levels. This includes newer fabrics like Propore and eVENT.
6. Garments made with air permeable fabrics, such as wind shirts and stretchwoven soft shell garments, give you more breathability and temperature regulation than fully waterproof garments. Use one in conditions where you don’t need the full protection of a waterproof garment.
Rain Shell Usage Guidelines
1. There is no performance substitute for choosing an appropriate shell and then using it intelligently. If you’re getting get too hot, too wet, or too cold, chances are there is something you can do to fix the problem. Adjust vents, remove or add insulating layers, or increase or decrease your hiking pace.
2. Make these changes quickly before a small problem becomes a big one. It takes a short time to soak a base layer and a long time to dry it out. In benign conditions (72 degrees and 33% relative humidity) it can take almost 8 hours to dry 8 oz of moisture from a damp wool baselayer, and it can take almost 14 hours to dry a completely saturated baselayer (if you were hanging these garments up to dry). In cooler and/or more humid conditions the top would take even longer to dry.
3. Without risking hypothermia, adjust your clothing layers and ventilation so that you are comfortably cool while exercising. Quickly put on a warmer layer when you stop to avoid excessive evaporative cooling.
4. When you begin to overheat (sweat), drop your exertion level some. Even a very slight decrease in pace (one quarter to one half mile per hour, or if you are climbing or hiking uphill, as little as one hundred vertical feet per hour) is often enough to keep you comfortable and dry.
5. In general, a well-vented shell will keep you drier than a poorly vented one. At high exertion levels, ventilation features such as pit-zips, full front zippers, and mesh-backed venting pockets are useful. Ventilation allows you to cool your body and to expel moisture rapidly.
6. Shell breathability is important. Second to ventilation, this is the shell property most important to keeping you dry. Some of the newer WP/B fabrics breathe exceptionally well relative to their older cousins, and significantly reduce moisture accumulation. Soft shells offer far better breathability and air permeability, with a wider level of temperature regulation, and in general, are more suited for high exertion levels in foul conditions where drenching or sustained rain is not an issue.
Smart use of your shell reduces moisture accumulation. Use the vents, regulate your exertion level, and adjust your insulation layers to prevent the accumulation of moisture in your clothing system. Even a good shell will get you wet if you don’t use it properly.
Rain Shell Selection Guidelines
There is no single shell that will work well in every environmental condition across all levels of exertion. Determine the temperature ranges, humidity, wind, precipitation, and levels of exertion you expect, and select your shell accordingly. For example, if you will be walking mostly in cooler temperatures with significant rain in the forecast, a waterproof breathable shell with a minimum of zippers (potential leakage points) and a good hood with a wire-stiffened brim might be your best choice. On the other hand, if you’re winter backcountry skiing in subfreezing temperatures where the only precipitation is dry snow, then a full-zip wind shirt or stretchwoven soft shell might be a better choice. Sometimes a combination of a wind shirt or a stretchwoven top, and a light rain jacket, is worth the weight for the flexibility the combination provides.
Here are some general guidelines:
1. Soft shells (including nylon and polyester wind shirts, and stretchwoven nylons) are the best choice for most conditions you will likely encounter, even in light precipitation. Their combination of water resistance and air permeability contributes to enhanced breathability and better temperature regulation, ideal performance criteria for moderate to high exertion activities in inclement conditions. Soft shell use requires some practice and skill to maintain comfort in inclement conditions. If you are uncomfortable or inexperienced using a non-waterproof shell in inclement conditions, consider a well-ventilated waterproof breathable shell instead.
2. If you decide you need a fully a waterproof shell and money is no object, spend the money on a new high performance breathable fabrics like Gore-Tex PacLite III, Gore-Tex XCR, eVENT, or one of the very breathable PU’s like Entrant G2 X2 (see Figure 1).
3. If money is a constraint, select garments made with low cost PU based fabrics. Make sure that you choose one with well designed ventilation! This will greatly increase your comfort at high exertion levels, since these fabrics are not as breathable as their more expensive competitors. Examples would include the Red Ledge Thunderlight Parka or the Marmot Precip Jacket.
4. If you want a waterproof breathable jacket, don't need the durability of a three-layer shell, but still demand a lightweight garment, consider some of the new PU shells in the 8 to 9 oz (225 to 255 g) range - like the Patagonia Specter Pullover (8.0 oz, 227 g), the Montane Superfly Jacket (2003 model, PU-based, 8.6 oz, 244 g), or the Montane Hydra-Lite Smock (7.6 oz, 215 g).
5. If you aren’t as concerned with durability, but want to enjoy the benefits of maximally breathable waterproof garments, try microporous polypropylene garments from Rainshield. Alternatively, consider an inexpensive wind shirt in combination with a Rainshield Propore jacket. The combination may weigh less than 8 oz and cost less than $50, but gives you the flexibility of an ultra breathable soft shell and one of the most breathable waterproof jackets on the market.